Problems caused by electromagnetic interference (EMI) have increased with the functional advancement and the prevalent use of electric and electronic devices. Electromagnetic noises are classified roughly into conduction noises and radiation noises. A conduction noise elimination method uses a noise filter. One radiation noise elimination method uses a metal case for electromagnetically isolating a space, another radiation noise elimination method inserts a metal sheet between wiring boards, and a third radiation noise elimination method coats a cable with a metal foil. These methods are effective in electromagnetically shielding circuits and power source blocks. But the shielding members used by those methods are opaque and hence are not suitable for suppressing the radiation of electromagnetic radiation from displays, such as CRTs and PDPs.
The PDP includes, in combination, a glass panel provided with data electrodes and a fluorescent layer, and a glass panel provided with transparent electrodes. The PDP generates intense electromagnetic radiation, near-infrared rays and a large amount of heat when the PDP operates. Generally, a front sheet is disposed on the front surface of the PDP to suppress the radiation of electromagnetic radiation from the PDP. The front sheet must be capable of reducing electromagnetic radiation of frequencies in the range of 30 MHz to 1 GHz by 30 dB or above. Near-infrared rays of wavelengths between 900 and 1,100 nm emitted from the front surface of the display can make other devices, such as VTRs, malfunction. Therefore, near-infrared radiation must be suppressed along with electromagnetic radiation. The front sheet must have proper transparency, namely, a visible light transmitting characteristic or visible light transmittance, to make images displayed by the display clearly visible.
An electromagnetic shielding sheet included in the front sheet for displays has a metal mesh. Lines forming the metal mesh of the front sheet for electromagnetic shielding must be barely visible and scarcely light-reflective, and must have proper transparency and brightness to make displayed images clearly visible. There have not been any metal meshes capable of effectively dealing with electromagnetic shielding, infrared shielding and visibility improvement, to say nothing of metal meshes capable of providing electromagnetic shielding and visibility improvement on a practical level. An electromagnetic shielding sheet manufacturing method is required to be capable of stably manufacturing accurate electromagnetic shielding sheets by a small number of steps.
To deal with both electromagnetic shielding and transparency, some conventional electromagnetic shielding sheets are provided with a transparent base sheet coated with a thin film of a transparent conductor, such as a metal or a metal oxide, and other conventional electromagnetic shielding sheets are provided with a metal mesh consisting of fine lines of an opaque conductor, such as a metal. However, the electromagnetic shielding sheet provided with a thin film of a transparent conductor cannot deal with both electromagnetic shielding and transparency. Therefore, the electromagnetic shielding sheet provided with a metal mesh has been used prevalently in recent years.
The electromagnetic shielding sheet provided with a mesh of an opaque conductor performs electromagnetic shielding by the opaque conductor lines of the mesh and exercises transparency to image light by the apertures of the mesh. The outer surfaces of the lines of the mesh are coated with a black coating to prevent the reduction of contrast in images due to the reflection of external light, such as sunlight, by the light-reflecting lines of the mesh. An electromagnetic shielding sheet disclosed in Patent document 1 to improve the visibility of a displayed image includes a base sheet, a transparent anchoring layer, and an electroless-plated layer of a mesh pattern, in which the transparent anchoring layer underlying the electroless-plated layer is changed into a black-patterned layer by electroless plating. A method disclosed in Patent document 2 forms a copper oxide film on the surface of a metal mesh included in an electromagnetic shielding sheet to suppress the reflection of external light. A method of forming a mesh with black lines disclosed in Patent document 3 forms a metal mesh for an electromagnetic shielding sheet by a photoresist process using a black photoresist and leaves the black photoresist after the metal mesh has been formed. An electromagnetic shielding structure disclosed in Patent document 4 is formed by laminating a plastic film provided with a patterned copper foil patterned in a geometric pattern by a photolithographic process to a plastic sheet. A method of fabricating an electromagnetic shielding sheet disclosed in Patent document 5 forms a resin mesh of a radiation-curable resin, forms a metal mesh by coating the resin mesh with a metal layer by electroless plating, and blackens the surface and side surfaces of lines forming the metal mesh by oxidation or sulfidation. The metal meshes (lines) of all the electromagnetic shielding sheets fabricated by the methods mentioned in Patent documents 1 to 5 are not sufficiently dark and are unable to make displayed images clearly visible in a place illuminated by external light.
An electromagnetic shielding sheet disclosed in Patent document 6 has a metal mesh formed by lines having blackened outer and inner surfaces and blackened side surfaces. A method of fabricating this electromagnetic shielding sheet forms a laminated sheet by laminating a copper foil to a transparent base sheet by means of an adhesive such that the blackened surface, treated by a blackening treatment, of the copper foil is bonded to the transparent base sheet. However, since the blackened surface is roughened by the blackening treatment, an unsatisfactory adhesive film, such as an irregular adhesive film or an adhesive film containing bubbles, is formed to spoil the quality of the laminated sheet. The lines forming the metal mesh are often broken by etching. Since the other surface of the copper foil and the side surfaces of the lines are subjected again to a blackening treatment after laminating the copper foil to the transparent base sheet, this method needs to perform the blackening treatment twice. Thus this method is troublesome and reduces yield. Since the surface, facing the display, of the mesh is coated with a black layer, the mesh absorbs light emitted by the display and the brightness of the display is reduced.
Patent document 1: JP 5-283889 A
Patent document 2: JP 61-15480
Patent document 3: JP 09-293989 A
Patent document 4: IP 10-335885 A
Patent document 5: JP 11-174174 A
Patent document 6: JP 2002-9484 A